Method and apparatus for controlling an immersed multisource array emitting acoustic impulses

ABSTRACT

In a multisource array of seismic sources, each source is associated with one or more sensors (Ci) for picking up the impulses (TB or signature) emitted by the associated source when triggered. The apparatus essentially includes one or more acquisition arrays (Ai), each one associated with a storage unit (Mi) for all the signals emitted by the pickups (Ci) under the control of a computer (10). Each memory unit includes a comparison module (19, 21, 22) for comparing the digitized samples with a threshold value. The number of each one of the channels where a threshold overrun has occurred is recorded in a reference register (24) which is systematically read by the computer. This monitoring of the level of the picked up signals is carried out outside the normal period where the sources are triggered.

BACKGROUND OF THE INVENTION

The object of the invention is to provide a method and an apparatus forcontrolling a multisource array emitting acoustic or seismic impulses,which makes it possible to detect possible uncontrolled triggerings.

Marine seismic prospecting methods are usually carried out by using awave emission array and a wave reception array towed by a ship along aseismic profiling plane to be studied. The waves generated by theemission array are reflected by different reflectors of the immersedformations and are received by the reception array which generallyconsists of a seismic streamer of great length along which a largenumber of sensors are arranged.

The emission array often consists of a plurality of impulse sourcestowed while immersed and connected with the ship by groups ofmultifunction cables or umbilicals. The form of the wave produced bythese impulse sources depends on the type of the sources. If the sourcesare of the explosion type, such as airguns for example, the main peak isproduced first. With sources of the implosion type such as waterguns,the main peak is preceded by a precursory peak with a lower amplitude.Shooting detectors are integrated into the sources or arranged close tothem in order to determine the triggering instants and/or the form ofthe produced impulses.

These sources are immersed, as the case may be, at substantially equaldepths or systematically at different depths. The triggering instantsare selected with precision in view of the particular layout of theemission array in the water in order to obtain a powerful anddirectional source. It is a matter of obtaining, by selecting thetriggering instants of the different sources, the phasing of theirrespective main peaks in a certain direction. The operation is generallycomplex because multiple parameters have to be taken into account.According to its type, its depth of immersion and its mechanical stateafter a more or less long use, the effective instant when the main peakof the source occurs may vary within notable proportions.

A shooting synchronizer or sequencer adapted for taking into account thedifferent parameters characterizing the emission array used is utilizedto control the emission sequences of an array of immersed seismicsources.

Systems where sequencers are used for controlling impulse sources aredescribed, for example, in U.S. Pat. Nos. 4,599,712, 4,693,336,4,718,045, 4,739,858, and 4,757,482 and in European Patent ApplicationsNos. 31,196 and 48,623.

Marine seismic prospecting cycles are sometimes disrupted because somesources release themselves at an untimely moment, for various reasonsgenerally due to leaks in the hydraulic control circuits. It may be asimple triggering delay in relation to the planned instant of a shootingsequence, or it may be new spontaneous triggering during the sameseismic emission-reception cycle. In both cases, the resulting impulseproduced by the emission array is disrupted, and the seismic recordingsobtained are distorted.

It is therefore useful that the operator be able to supervise therunning of the emission array provided that this supervision does notmake the control management allotted to the shooting synchronizer usedexcessively complicated.

SUMMARY OF THE INVENTION

The method and the apparatus according to the invention make it possibleto take into account the tasks of checking a seismic emission array andto integrate it simply into the operations achieved by a shootingsynchronizer, notably that which is described in co-pending FrenchPatent Application No. FR 90/08,267.

The method applies to the control of an emission array comprising aplurality of acoustic impulse sources associated with acoustic wavedetectors arranged close to the sources piloted by a device controllingthe shooting sequences including a programmable computer. The methodcomprises the acquisition of signals picked up by said acoustic wavedetectors during a first time interval containing the planned triggeringinstants of the different sources and at a first sampling frequency.

The method comprises:

acquiring the signals picked up by the same detectors during a secondtime interval after the first one and at a second sampling frequencylower than the first frequency;

detecting the signals picked up during the second time interval by eachone of the detectors, whose level exceeds a set threshold value, and thestorage of the possible overruns in an overrun indicator;

reading systematically the overrun indicator by the computer; and

checking the signals which have caused an overrun of the set thresholdvalue.

The shooting check apparatus according to the invention makes itpossible to check the triggering of the sources of an immersed emissionarray consisting of a plurality of acoustic impulse sources associatedwith acoustic wave detectors arranged close to said sources. Theapparatus comprises an acquisition device connected to the differentdetectors for sampling and digitizing the signals produced by thedetector when said sources are triggered, a shooting control apparatusfor controlling the triggering of the sources, and a programmablecomputer communicating with the acquisition apparatus and with theshooting control apparatus, the computer being adapted for piloting theacquisition apparatus during a first time interval at a first samplingfrequency.

The apparatus comprises a detection assembly communicating with thecomputer and consisting of:

a comparison means for comparing each sample of all the signals receivedby the different detectors with a threshold signal imposed by thecomputer during a second time interval; and

a storage means for keeping the threshold overrun indications detectedby the comparison means for each source and until the end of eachemission-reception cycle,

the computer being adapted for reading the storage means to identify thedetectors which have picked up signals exceeding the threshold valueand, when requested by the operator, for reproducing each signal thathas exceeded the threshold value.

According to one embodiment, the total detectors are, for example,divided into groups of p detectors, the acquisition apparatus comprisesfor each group of p detectors means for multiplexing the signals comingfrom the different detectors and means for digitizing the multiplexedsignals, at least one storage unit for said digitized signals associatedwith a register for the digitized data to be successively stored and anaddress register, and the detection array comprises a comparatorproducing a binary comparison signal, a switching means controlled bythe address register to direct each binary comparison signal selectivelytowards the binary elements of a register with n bits, and the computeris adapted for sequentially reading the content of the binary elementsof the register with p bits to detect those corresponding to overruns ofsaid threshold value.

Each detection array comprises, for example, a register connected withthe computer to store a threshold value, this register being connectedwith said comparator.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the method and the device according tothe invention will be clear from reading the description hereafter of anembodiment given by way of non-limitative example, with reference to theaccompanying drawings in which:

FIG. 1 diagrammatically shows a marine seismic emission-reception array;

FIG. 2 shows the approximate form of an impulse emitted by a source suchas an airgun;

FIG. 3 shows the approximate form of a corresponding impulse emitted bya watergun;

FIGS. 4A to 4E show waveforms of an example of an emission-receptioncycle;

FIG. 5 shows the block diagram of a synchronizer controlling theprogress of the shooting sequences; and

FIG. 6 shows the block diagram of an acquisition and storage arraymaking it possible to supervise the sources.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A marine seismic emission-reception system, such as that schematized inFIG. 1, comprises an acoustic impulse emission array 1 towed inimmersion by a ship 2, as well as a reception array consisting of aseismic streamer 3 of great length. Emission array 1 generally comprisesseveral sources divided among several sub-arrays, each one consisting ofseveral sources located along a harness of multifunction cables orumbilicals 4. The sub-arrays are towed at the same depth with differentlateral offsets in relation to the trajectory of the ship and/or, as thecase may be, at different depths.

Each source receives a triggering signal from a control system 5 throughtransmission lines in the umbilical 4 that connects the source with theship. In return, control system 5 receives, from a kinematic sensorintegrated in each source, a triggering signal TB defining the exacttriggering instant. An acoustic sensor located in close proximity toeach source (one meter for example) or, as the case may be, to eachgroup of sources when several of them are grouped together, produces aproximity signal NF that is usually called "signature" SGN, which isalso transmitted to the control system 5. The same sensor is also usedbetween the shootings to measure the depth of immersion of each source.The signals TB are a few milliseconds earlier than the correspondingsignals SGN.

An initial instant ODT defines the beginning of each "shooting" sequence(FIGS. 2, 3). Each source is effectively triggered by control system 5with a delay TRET in order to obtain a phasing of the different impulsesproduced. Each source reacts to the triggering signal with a delay timeTRES depending on its mechanical and/or hydraulic structure.

After its triggering ordered with the time from the delay time intervalTRET (instant t1), an explosive source (FIG. 2) produces its main peak(instant t2) which is followed by a secondary peak with a loweramplitude (instant t3).

The main peak of a source of the implosion type (FIG. 3) is preceded atthe instant t2 by a precursory peak. The gap (t3-t2) is thepseudo-period TPSP which varies according to the well-known Raleigh'srelationship.

The control system 5 comprises (FIG. 5) an acquisition and storageapparatus 6 receiving the signals TB and/or SGN supplied by the sensorsassociated with all the sources C of the emission array. It alsocomprises a shooting control apparatus 7 for adapting the triggeringsignals transmitted to the solenoid valves of the different sources. Theacquisition and storage device 6, as well as control apparatus 7,communicate through an address and data bus 8 with an interface card 9which is connected with a programmable computer 10. Interacting betweenthe operators and computer 10 is achieved by means of a control desk 11and a display screen 12.

This use of a specialized apparatus makes it possible to obtain a higheracquisition, digitizing and storage speed than if computer 10 wereentrusted with these tasks.

Acquisition apparatus 6 (FIG. 6) comprises several acquisition cards Ai,each one having a set number p of channels making possible theacquisition of the signals TB or SGN picked up by p detection sensors(p=32 for example). Each channel comprises a preamplifier 13a . . . 13pin series with a bandpass filter 14a . . . 14p. The gain and features ofeach filter are adapted to the type of sensors Cl . . . Cp that areused. The filtered signals are applied to a multiplexer 15 with p inputswhose output is connected with a digitizing circuit 16. The signalsdigitized by each acquisition card Al . . . An are respectively storedin memory cards. The computer can read them by means of interface card9. The acquisition cards are controlled by means of a synchronizationcard 17 connected with bus 8 which synchronizes through signals SA andSM the multiplexing, the digitizing and the storage of the digitizeddata. The acquisition time and the sampling frequency may be different,depending on whether the sensors connected with the inputs of eachacquisition card are signals TB or SGN.

Each memory unit Mi comprises a memory module 18 associated with a dataregister 19 which is connected to the output of the analog to digitalconverter 16, and to an address register 20 controlled by thesynchronization signal SM. The inputs of a digital comparator 21 arerespectively connected to data register 19 and to a register 22 which islinked with bus 8. The computer loads into register 22 a digital wordrepresenting a threshold value VS. When each digital word passingthrough data register 19 is larger than the threshold value VS,comparator 21 emits an overrun bit. This bit is applied at the input ofan electronic switch 23 with p outputs which are respectively connectedto the inputs in parallel of a register 24 with p bits also. Switch 23is also controlled by the synchronization signal SM. The reset input RAZis connected to bus 8. Register 24 is reset at the end of each readingcycle. With this layout, sensors Cl to Cp being interrogated insequence, the different bits of register 24 are respectively associatedwith the different acquisition channels (13, 14). If on any channel anacquired sample exceeds the threshold value stored in register 22, alogical 1 is loaded into the corresponding binary element of register24. Computer 10 may, therefore, read at any time the different bits ofthe binary word in register 24 in order to test them and to know whethera threshold overrun has been noticed during an emission-reception cycle.

Each working cycle of the check device comprises the piloting of anemission sequence through the total sources of the emission array, areal time acquisition stage directly managed by the acquisitionapparatus and a measuring and checking stage where the computer, fromthe digitized data stored during the previous stage, authenticates thereceived signals TB or SGN, by comparison with reference data,calculates their reception time and updates the reference data byincluding authenticated data, as described in the above cited co-pendingFrench Patent Application No. FR 90/08,267.

Each emission-reception cycle begins at an initial instant marked by animpulse DS which is controlled by the operator or which occursautomatically at defined intervals (FIG. 4B). The impulse DS has theeffect of initializing all the counters and registers of the device.From the instant defined by DS, the computer orders the acquisition ofthe signals SGN indicative of the depths of immersion of the differentsources (FIG. 4C) and, in the case where the sources are of theimplosion type, it calculates their respective pseudo-periods in orderto determine the instants where they will have to be triggered to obtainthe phasing of their main peaks in a chosen particular direction.

The reference instant common to all the elements of the device isdefined by an impulse ODT (FIG. 4B). The acquisition of the signals TBand/or SGN picked up by the detectors begins from this instant. Theirstorage is triggered after 10 ms for example (FIG. 4D) at the same timeas the sequence of triggering of the different sources begins (FIG. 4E).

The acquisition and storage stage is divided into two parts. In a firstpart of duration T1 (FIG. 4D), the signals TB and SGN are sampled andstored at a frequency f1 of 10 kHz for example (signal SM in FIG. 6).This main signal acquisition interval T1 is followed by a supervisiontime interval T2 which goes on until the following initializationimpulse DS is emitted. During this second interval, the device continuesto sample and store the signals TB and SGN at a lower frequency f2, of 3kHz for example. No signal TB or SGN should normally be detected outsideperiod T1. Any signal detected during this interval T2 correspondstherefore to a spontaneous triggering of the sources or it is aparasitic signal.

As described in the above cited French Patent Application No. FR90/08,267, the computer 10 achieves different operations ofauthentication of the picked up signals TB and SGN and of calculation ofthe respective triggering times of the sources.

With the layout described in FIG. 6, the computer can also, at any time,test the different bits of the state register 24. If, during the periodT2 of the cycle, signals due to self-triggering of the sources or tointerference have been picked up, register 24 indicates the channel(s)concerned. The operator can then, by means of his desk 11 (FIG. 5),control the display of each one of the abnormal signals. A visualexamination will tell him if it is a spontaneous triggering.

We claim:
 1. A method for checking the triggering of an immersedemission array including a plurality of acoustic impulse sources, aplurality of acoustic wave detectors arranged close to said sources, anda device for monitoring the triggering sequences of the sourcesincluding a programmable computer, wherein the acquisition of signals bysaid acoustic wave detectors is effected during a first time intervalcontaining the expected triggering times of the acoustic impulsesources, said method comprising:acquiring the signals by said acousticwave detectors during a second time interval subsequent to the firsttime interval; detecting the ones of the signals acquired during thesecond time interval whose levels exceed a set threshold value aspossible overruns; storing the possible overruns in an overrunindicator; reading systematically the overrun indicator; and checkingthe signals which have caused the overruns.
 2. A shooting checkapparatus for checking the triggering of the sources of an immersedemission array including a plurality of acoustic impulse sources, aplurality of acoustic wave detectors arranged close to said sources, anacquisition apparatus connected to the detectors to sample and digitizethe signals produced by the detectors when the sources are triggered, ashooting control apparatus for controlling the triggering of thesources, and a programmable computer communicating with the acquisitionapparatus and the shooting control apparatus, the computer being adaptedfor controlling the acquisition apparatus during a first time intervalat a first sampling frequency the shooting check apparatus comprising:adetecting assembly communicating with said computer and including: a)comparison means for comparing each sample of signals received by thedetectors with a threshold value from the computer during a second timeinterval and generating a threshold overrun indication when a receivedsample exceeds the threshold value; and b) storage means for storingthreshold overrun indications from the comparison means for each sourceduring each emission-reception cycle, the computer being adapted forreading the storage means to identify detectors that have picked upsignals exceeding the threshold value and being responsive to a requestby an operator for reproducing each signal exceeding the thresholdvalue.
 3. A device as claimed in claim 2 wherein the detecting assemblycomprises a plurality of groups of p detector means; the acquisitionapparatus comprises for each group of p detector means multiplexingmeans for multiplexing signals coming from the detector means,digitizing means for digitizing the multiplexed signals, register means,at least one memory unit for storing digitized signals associated withthe register means, an address register, a comparator for producing abinary comparison signal, a p-bit register, switching means controlledsimultaneously with the address register for directing each binarycomparison signal selectively towards the binary elements of the p-bitregister, said computer being adapted for reading sequentially thecontents of the p-bit register to detect those elements storing bitsindicative of overruns of said threshold value.
 4. An apparatus asclaimed in claim 3, wherein each detecting assembly further includes athreshold register connected to the computer and to the comparator forstoring a threshold value.
 5. An apparatus as claimed in any one ofclaims 2 to 4, wherein the acquisition apparatus comprises at least oneacquisition array including a plurality of amplification and filteringunits, storage means, multiplexing means connecting the amplificationand filtering units to the storage means, and an address and data busfor permitting the acquisition apparatus and the shooting controlapparatus to communicate with the computer.